1. Field of the Invention
This invention generally relates to illumination apparatus, and particularly to an arrangement of light reflectors for efficiently using light emitted from a single light source to uniformly illuminate an original. The present apparatus is particularly useful in illuminating originals in a photographic printer, although the apparatus is equally adapted for use with other types of copiers, e.g., electrographic copiers.
2. Description Relative to the Prior Art
Apparatus for illuminating an original generally falls into two categories, each adapted to achieve the same result by a different engineering approach. One approach involves illuminating a slit through which the original is line-imaged by an optical system upon a photosensitive surface. Either the original moves past the illuminated slit or the slit moves over a stationary original to achieve the same result. In both cases the image is built up line by line on the photosensitive surface. The other approach involves illuminating an entire copy plane in which the original is positioned. The latter approach requires at least a momentary phase in the copy cycle when all parts of the original may be simultaneously imaged on the photosensitive surface, i.e., the image of the document is at least momentarily stationary with respect to the photosensitive surface. For example, a short exposure of the original effectively "freezes" motion with respect to a moving photosensitive belt.
Exemplary of the first approach is U.S. Pat. No. 3,982,116 which describes a single light source and an arrangement of mirrors for reflecting light through a slit defined transversely to a copy platen. The mirrors surrounding the light source are designed to converge light rays to a line focus beyond the slit. By placing an original on the platen, and moving the platen relative to the slit, the converging light rays scan the surface of the original.
The second approach is illustrated by copy machines described in each of U.S. Pat. Nos. 3,586,849 and 3,777,135. In each machine, an original is placed upon a transparent support platen mounted relative to an illumination lamp assembly. In order to provide homogeneous illumination, four lamps are orthogonally spaced around each edge of the original. By means of reflectors with compound surfaces (i.e., both planar and curved surfaces), light rays emitted from each lamp are reflected upon the surface of the original such that they overlap and complement rays from the opposing lamp. The light rays reflected from the surface of the original produce image light corresponding to the informational areas on the original. The image light traverses an optical system and exposes the photosensitive surface of a flexible photoconductive belt arranged on a belt assembly.
The application served by these conventional approaches involves substantially opaque material situated in a copier such that light reflects from the opaque surface to a photosensitive surface. Unlike a transparency, which may be illuminated from a light source substantially on the optical axis of the copier, the light source for reflection copying is offset from the optical axis; otherwise the illumination source would block the image from reaching the photosensitive surface. In practice, because uniform illumination is required, it is common to use several light sources. Each source is spaced from one side of the opaque original.
Illumination of an original using a plurality of light sources is a satisfactory approach in some applications. However, in other applications this approach has significant limitations. For example, in a photographic printer where an image is projected from a color print to photosensitive color paper, the spectral characteristics of each lamp must be considered in arriving at proper exposure times and color filtration for a given paper. Not only do many lamps differ initially in spectral distribution, but lamp aging causes the distribution for each lamp to change at an individual rate.
The potential for unwanted color variability therefore increases as the number of light sources increase. Matching the lamps with proper filters to maintain a proper spectral distribution is difficult at best. To worsen matters, the burden is thrown upon the photofinisher rather than the manufacturer. Each light source would require its own set of color filters to control the spectral characteristic of light from that source. The mere number of components that must therefore be matched becomes unmanageable.
U.S. patent application Ser. No. 930,287 in the names of A. M. Boone, J. F. Bloemendaal and A. R. Zanolli, filed concurrently with and assigned to the same assignee as the present application, describes illumination apparatus which effectively reduces the aforementioned problems by providing uniform illumination of an original with a single flash lamp. A pair of parabolic reflectors partially surround the lamp and direct two separate collimated bundles of light toward opposite sides of the original. A planar reflector positioned in the path of each collimated bundle of light intercepts and reflects the collimated light rays upon the original to uniformly illuminate the original from opposite sides.
One problem with illumination apparatus such as described in the copending application Ser. No. 930,287 is that some light emanating directly from the lamp may strike the original without reflecting from any of the reflectors in the system. This condition interferes with uniform illumination of the original and, with certain types of originals, causes specular reflections through the optical system to the photosensitive paper. These reflections show up as blotches on the copy print.
Consequently, the illumination apparatus described in the copending application also included a light baffle positioned between the light source and the original to substantially prevent light rays from passing directly between the source and the original. While the baffle provides the effect desired, a portion of the light energy generated by the lamp is thereby lost to the system.